This invention relates to an improvement in an electrically heated transparency such as may be employed in a vehicle to provide defrosting, deicing, or defogging capability. In particular, the improvement is in the bus bar arrangement and is intended to provide more reliable performance of the heating means.
It has been know to pass electric current through a transparent conductive coating on a transparency in order to raise the temperature of the transparency. Generally, a source of electrical potential is connected to the conductive coating by way of a pair of bus bars along opposite sides of the area of the transparency to be heated. The bus bars have low resistivity relative to the coating and are intended to distribute the current evenly over the area to be heated. The bus bars may be comprised of metallic foil strips, but in the case of glass transparencies they preferably are comprised of a metallic-ceramic frit material fused onto a surface of the transparency. A typical arrangement includes bus bars configured as substantially parallel stripes on opposite sides of the heated area, with electrical leads attached to each bus bar and extending away from the opposite edges of the transparency as shown in U.S. Pat. Nos. 4,323,726 (Criss et al.) and 4,668,270 (Ramus). Locating the leads on the same side of the transparency and preferably closely adjacent to each other is advantageous for the sake of easier installation of the transparency in the vehicle and simplifying the connection with the electrical power source. Therefore, U.S. Pat. No. 3,895,213 (Levin) and U.S. Pat. No. 4,543,466 (Ramus) provide an extension of one of the bus bars around an end of the transparency so that connections to both bus bars can be made in one relatively compact area.
Several modes of failure of heated transparencies can involve the bus bars. An imperfection in or damage to a bus bar such as a thin area or crack results in a concentrated flow of current through that area which in turn causes overheating. A gap in the circuit can cause arcing. Overheating or arcing can damage the plastic innerlayer in a laminated transparency, or even damage the glass in severe cases. Excessive localized heat in a bus bar can also exacerbate the defect, leading to a breach in the circuit and possible inoperativeness. A complete break in a bus bar would, of course, severely impare or render inoperative the heating circuit. Heated transparencies are even more vulnerable to bus bar problems in those cases such as in the aforesaid U.S. Pat. Nos. 3,895,213 and 4,543,466 where an extension of one of the bus bars extends around a third side of the transparency to provide adjacent terminal locations. Not only is there a greater length of bus bar subject to failure in such a case, but also failure in the extension portion is particularly critical since it carries a heavy power load and its malfunction renders the entire system inoperative.